The present invention relates to an optical disk of a single-spiral land/groove configuration, wherein information is recorded on lands and grooves (hereinafter referred to as land/groove recording), land tracks and groove tracks alternate to form a single spiral.
The invention also relates to an optical disk drive device using such an optical disk.
In conventional phase-change type optical disks, data is recorded only on grooves, and lands serve to guide the light spot for tracking, and to reduce crosstalks from adjacent groove tracks. If data is recorded on lands as well, the track density can be doubled on condition that the width of the grooves and the width of the lands are both unchanged. It has has been discovered that the crosstalk between adjacent land and groove tracks is reduced if the difference in height between the lands and grooves is .lambda./6 (.lambda. being the wavelength of the light source). Owing to this discovery, the use of both of the land and groove tracks has become feasible. The use of both land and groove tracks is also advantageous with regard to the ease of mastering of the disk: it is easier to attain a certain recording density by the use of both land and groove tracks than by reducing the track pitch using only the groove tracks.
For instance, in the case of optical disks for use as computer data files, optical disks in which data is recorded both on land and groove tracks, and the tracks are concentric, so that after recording of one revolution (on a groove track, for example), a track jump is effected to start writing on the adjacent track (a Land track). Sectors are managed in accordance with the sector addresses. Accordingly, the operation for recording and reproducing data, such as computer data, which need not be continuous, can be carried out without difficulty.
Rewritable optical disks are however also used for recording continuous data such as motion picture, or music. In multimedia applications (where computer data and video and audio data are mixed), spiral tracks, as in compact disks, may be preferred because of the continuity of the tracks. For instance, FIG. 18 shows a track pattern of a disk for recording on land tracks and groove tracks in the prior art. Groove tracks 1 and land tracks 2 between adjacent groove tracks 1 are divided into information recording sectors by header parts 5b, each sector forming a unit for recording data. The groove tracks and Land tracks may sometimes referred simply as "grooves" and "lands," respectively.
With the configuration shown in FIG. 18, all the groove tracks 1 form a single spiral, and all the land tracks 2 form another spiral. For recording or reproduction, the light beam traces from the beginning (inner end) of the spiral formed of the groove tracks, for example, and upon arrival at the end (outer end) of the spiral, the light beam jumps to the beginning of the other spiral formed of the land tracks. Switching between the groove track spiral and the land track spiral requires access between the inner and outer peripheries of the information recording region of the disk, and therefore a certain time delay is inevitable.
The information recording region of the disk may be divided into annular zones, so that the length over which the light spot must jump for switching between the groove track spiral and the land track spiral is shortened to the distance between the outer and inner peripheries of the annular zone. However, there is still a considerable time delay for the jump.
FIG. 19A and FIG. 19B show details of the header portion in a conventional optical disk wherein data is recorded on both groove and land tracks. FIG. 19A shows the case where headers 5b are provided separately for the land and groove tracks, and addresses dedicated to the sectors in the respective tracks are formed. FIG. 19B shows the case where headers 5b are provided on an extension of a boundary between land and groove tracks, and each address is shared by the sectors in the land and groove tracks separated by the boundary. In either case, the headers include address pits.
The header portion 5b is formed of embossments (dents or projections) physically formed for representing the address information arid the like of the sector preceded by the header. Specifically, pits having the same height as the lands, or pits having the same depth as the grooves are formed in the header portion where no tracks are formed.
There are several methods for forming prepits suitable for the land/groove recording configuration. Two principal ones are those shown in FIG. 19A and FIG. 19B. In the configuration shown in FIG. 19A, dedicated prepits are provided for each sector of the land or groove track. Because the dedicated prepits can record various items of information, such as the one indicating whether the sector following the dedicated prepits is a land track sector or a groove track sector, control in the optical disk drive device is facilitated. However, the width of the prepits must be sufficiently narrower than the track width. This means that the laser beam used for forming the tracks cannot be used for forming the prepits, but a Less powerful laser beam must be used for the formation of the prepits, and the fabrication of the medium is difficult.
In the configuration shown in FIG. 19B, the prepits are shared by the land and groove tracks adjacent to each other. The prepits can be formed by the using the same laser beam used for forming the tracks, and by shifting the laser beam by half a track pitch laterally of the track, i.e., in the radial direction of the disk. However, during writing or reading of the disk, the shared prepits cannot indicate whether the sector following the prepits is in a land track or in a groove track, so that the optical disk drive device must have a means to find whether a land track or groove track is being traced by the light spot, and the control in the optical disk drive device is difficult.
In the above-described optical disk allowing recording and reproduction, it is also necessary to solve the problem of the track offset. This relates to the fact that the one beam-and-push-pull method is used for the tracking, rather than a three-beam method. This is because the recording requires a greater laser power. Also, in pit-forming recording on a write-once disk or the like, the side spots (used in a three-beam method) causes a disturbance to the tracking operation.
In a push-pull tracking, the tracking error is detected using the diffraction distribution of the light spot illuminating the pregrooves as shown in FIG. 20, and fed to the servo system. More particularly, an optical head 8 has a laser diode 60 emitting a laser beam, which is passed through a half-mirror 61 and an objective lens 62 to illuminate an optical disk 7 rotated by a disk motor 64. The reflected light beam from the light spot on the disk 7 is guided by the objective lens 62 and the half-mirror 61 and is received by a photodetector 16, and the tracking error is detected using the diffraction distribution of the light spot on the optical disk 7. The detected tracking error is used to control an actuator coil 63 for driving the objective lens 62.
For instance, a tilting of 0.7 degrees or an eccentricity of a 100 .mu.m (equivalent to lateral movement of the objective lens 62 of 100 .mu.m as indicated by broken lines in FIG. 20) causes shifting of a light distribution 17 on the photodetector 16, and an offset of 0.1.mu.. To prevent such a phenomenon, a drive device having higher mechanical and optical accuracies is used, and various other contrivances are adopted.
FIG. 21A shows the header part 5b having a mirror surface part 6 for track offset correction. FIG. 21B shows the header part 5b having wobble pits 58 and 59 for track offset correction. The wobble pit pits 58 and 59 are shifted in the radial direction by 1/2 of the track pitch. These methods are described in the following publications: (1) Ohtake, et. al. "Composite Wobbled Tracking in the Optical Disk System," on pp. 181-188 in Optical Memory Symposium '85, held on Dec. 12-13 in 1985, published by Optical Industry Technology Promotion Association, (2) Kaku, et al. on "Investigation of compensation method for track offset," pp. 209-214 in Optical Memory Symposium '85, held on Dec. 12-13 in 1985, published by Optical Industry Technology Promotion Association.
FIG. 22 shows a track offset correction circuit used in combination with a disk having the mirror surface portion 6 shown in FIG. 21A. A split photodetector 16 detects the tracking error by a push-pull method. An adder 11 adds the outputs of the two half-portions of the split, photodetector 16 to produce a signal indicative of the total amount of light received, which corresponds to the total amount of light reflected from the disk. A differential amplifier 12 determines the difference between the outputs of the two half-portions of the split photodetector 16, to produce a signal indicative of the tracking error. A mirror surface detector 13 detects the mirror surface portion 6. A sample-hold circuit 14 samples arid holds the tracking error signal when the light spot passes the mirror surface portion 6, and holds the sampled value as an offset information. A differential amplifier 15 determines the difference between the tracking error signal and the offset information. The output of the differential amplifier 15 indicates the tracking error having the offset removed.
FIG. 23 shows an offset correction circuit used in combination with a disk having wobble pits shown in FIG. 21B. A wobble pit detector 18 receives the output of the adder 11, and detects the wobble pits, i.e., produces a signal to a sample-hold circuit 19 when the light spot passes the wobble pit laterally shifted toward one side of the track, and produces another signal to a sample-hold circuit 20 when the Light spot passes the wobble pit laterally shifted toward the other side of the track. Responsive to these signals (i.e., when the light spot passes the wobble pits 58 and 59), the sample-hold circuits 19 and 20 sample the output of the differential amplifier 12, and holds the sampled values. A differeintial amplifier 21 determines the difference of the outputs of the sample hold circuits 19 and 20, as an offset. An adder 50 adds the tracking error signal obtained at the differential amplifier 21 to the tracking error signal obtained by means of the ordinary push-pull method, to produce the tracking error signal from which the offset has been removed.
FIG. 24 illustrates the control characteristics for the case where a tracking error signal obtained by wobble pits and the tracking error signal by means of the conventional push-pull method are both used. G1 represents a tracking open loop characteristic by means of the conventional push-pull method, and G2 denotes a tracking open loop characteristic by means of the wobble pits.
In the configuration shown in FIG. 21A, at the mirror surface portion 6, the guide grooves are discontinuous or interrupted. With this configuration, a correction circuit for correcting the mirror surface offset, shown in FIG. 22, is used. The signals output from the two half-portions o the split photodetector 16 are input to the differential amplifier 12, which thereby produces a tracking error signal. On the basis of the sum signal produced by the adder 11, the mirror surface detector 13 generates a timing signal indicating the timing at which the light beam is passing the mirror surface portion 6. The tracking error signal .DELTA.T produced by the differential amplifier 12 includes an error component .DELTA.Tg due to the shift of the objective lens, a true tracking error .DELTA.Ts, and an offset component .delta. due to various causes including the tilting of the disk, so that it is given by: EQU .DELTA.T=.DELTA.Ts+.DELTA.Tg+.delta. (1)
The sample-hold circuit 14 samples the tracking signal at the mirror surface portion 6, and holds the sampled value. The output of the sample-hold circuit 14 represents .DELTA.Tg+.delta.. Accordingly, in view of the equation (1), subtracting the output of the sample-hold circuit 14 from the output of the differential amplifier 12 at the differential amplifier 15 during the scanning of the data sectors results in the true tracking signal .DELTA.Ts. In this way, a closed-Loop servo system for achieving an accurate track following can be formed.
Another method of correction is a method using wobble pits. According to this method, wobble pits shifted in opposite directions as shown in FIG. 21B are formed by alternately deflecting the light beam, using ultrasonic deflector, during fabrication of the original disk for mastering. During recording and reproduction, the outputs of the differential amplifier 12 when the light spot is passing the wobble pits on the respective sides are compared, to detect the tracking error. Specifically, a differential amplifier 21 shown in FIG. 23 determines the difference between the outputs of the sample-hold circuits 19 and 20 to obtain the tracking error. As shown in FIG. 25, when the light spot passes along a line closer to the center of the pit 58 on one side (upper side in FIG. 21B) than to the center of the pit 59 on the other side (lower side in FIG. 21B), an output signal illustrated by the dotted line is obtained. When the light, spot passes along a line closer to the center of the pit 59 on the lower side than to the center of the pit 58 on the upper side, an output signal illustrated by the solid line is obtained. The difference obtained by subtracting the output of the differential amplifier 12 obtained when the light spot is passing the wobble pit 59 at the back or one one side, from the output of the differential amplifier 12 obtained when the light spot is passing the wobble pit 58 at the front or on the other side, represents the magnitude of the tracking error and the direction of the tracking error. This means that the position at, which the true light spot passes is detected. Compared with the method relying on the diffraction distribution due to pre-grooves, the above-described method realizes a better servo system.
Another tracking method has been proposed, in which the feature of the above-described wobble pit method is maintained, and which is compatible with systems using conventional push-pull tracking method. The sector configuration in this system is composed of an index field with pre-pits shown in FIG. 21B, and user data field. The index field is provided with address information, as well as wobble pits which may or may not serve also as a sector detection mark, and pre-grooves for tracking. With such a configuration, the true tracking error is detected from the wobble pits, and the offset used in push-pull tracking can be corrected. In this case, the open-loop characteristic of the tracking servo is such that the gain for tracking on the basis of the wobble pits is relatively large in the low-frequency region, and the gain for the tracking on the basis of the push-pull method is relatively large in the high-frequency region, as shown in FIG. 24. As a result, data can be recorded and reproduced, while the light spot is maintained on the center of the track, regardless of the drive device used, and compatibility between the recorded disk and the drive device can be preserved.
With the above-described optical disk device, information is recorded on lands and grooves to increase the recording density. One way of recording continuous information, such as video and audio information, on lands and grooves in an optical disk, is to connect each revolution of land with each revolution of adjacent land, so that recording track alternate between Land and groove every revolution. In such a configuration, the polarity of tracking error signal has to be reversed every revolution. If the connecting point is not correctly detected, the servo operation may be disturbed, and tracking may fail.
In particular, such an offset is due to the error in the mounting of the tracking sensor in the optical head, and a tracking offset due to stray light, and the offset due to these factors are reversed when the polarity of the tracking error signal is reversed.
When the connecting point is not detected due to the eccentricity of the disk or vibration of the disk, the light spot may deviate to a next track, or tracking may fail.